1. Field of the Invention
The present invention relates to an image forming apparatus in which a latent image formed on the surface of an image transport member such as a photosensitive drum is developed by toner transported by a toner transport member such as a developing roller.
This type of image forming apparatus generally uses a mono-component developer which is comprised only of toner containing no carrier particles. It is necessary in order to obtain excellent development quality by using such a mono-component developer to prevent variation of the amount of toner supplied to the image transport member from the toner transport member.
Particularly, in an electrophotographic apparatus designed to form a full-color image by superimposing a plurality of toner images of different colors, if the amount of toner supplied varies from color to color, the color reproducibility is deteriorated, and the image quality is considerably degraded.
2. Description of the Related Art
In an electrophotographic apparatus or other similar image forming apparatus, while toner is being transported in a state of adhering to the surface of a developing roller as a toner transport member, a layer of toner is formed to a predetermined uniform thickness by the action of a layer thickness regulating blade that is disposed in pressure contact with the surface of the developing roller.
The toner layer is transported to the surface of a photosensitive drum as an image transport member by the rotation of the developing roller, and a latent image formed on the surface of the photosensitive drum is developed by the toner. At this time, an amount of toner required for development is ensured by setting the peripheral velocity of the developing roller at a higher level than that of the photosensitive drum.
The amount M of toner Supplied to the photosensitive drum from the developing roller is given by EQU M=(Vd/Vp)Dt.a.P.nd
where Vd/Vp: the peripheral velocity ratio (the peripheral velocity of the developing roller/the peripheral velocity of the photosensitive drum) PA1 Dt: the toner layer thickness PA1 a: the toner density PA1 P: the toner packing rate PA1 nd: the development efficiency (the ratio of the amount of toner transported from the developing roller to the photosensitive drum to the amount of toner attached to the developing roller)
Accordingly, it has heretofore been considered from the above expression that the amount of toner supplied to the photosensitive drum can be maintained at a constant level by setting the peripheral velocity ratio such that when the toner layer is relatively thin, the peripheral velocity ratio is set at a relatively high level, whereas, when the toner layer is relatively thick, the peripheral velocity ratio is set at a relatively low level. Thus, the conventional practice has been to set the toner layer thickness and the peripheral velocity ratio in an appropriate relationship, considering only the relationship therebetween, at the time of designing an image forming apparatus.
However, the development efficiency, that is, the ratio of the amount of toner moving to the photosensitive drum to the amount of toner attached to the developing roller, varies according to the amount of toner charge.
Accordingly, when an apparatus is designed by taking into consideration only the relationship between the toner layer thickness and the peripheral velocity ratio, the amount of toner supplied to the photosensitive drum varies with the fluctuation of the development efficiency due to the variation of the amount of toner charge, thus degrading the development quality, particularly the color reproducibility in the case of full-color development.